1. Field of the Invention
The present invention relates to an ink cartridge and an ink jet recording apparatus and particularly to an ink cartridge and an ink jet recording apparatus each of which assures that ink is ejected with stability as an amount of ink remaining in an ink storing portion decreases.
2. Discussion of Related Art
There is known an ink jet recording apparatus in which ink is supplied from an ink cartridge to a recording head and each of a plurality of nozzles of the recording head ejects a droplet of the ink to record an image on a recording medium such as a sheet of paper.
For example, Japanese Patent Publication No. 61-233551 or its corresponding U.S. Pat. No. 4,719,475 discloses an ink cartridge including an ink bag which stores ink, and an ink jet recording apparatus including the ink cartridge. The ink bag is obtained by bonding flexible sheets to each other, and accordingly has a flat configuration. The flat ink bag is accommodated in the ink cartridge, such that the flat bag takes an upright posture in which a widthwise direction of the flat bag is parallel to a vertical direction. In this state, the ink cartridge is connected to the recording head.
As the recording head repeats recording operations, the amount of ink in the ink bag gradually decreases and accordingly the ink bag gradually shrinks and becomes thinner. Therefore, in the case where the ink bag is used in the above-indicated state in which the widthwise direction thereof is parallel to the vertical direction, a height position of a level of the ink in the ink bag moves in a vertically downward direction as the amount of ink decreases and accordingly a shape of the ink bag changes.
When the height position of the ink level of the ink bag changes, a difference between the height position of the ink level and a height position of the recording head or, in other words, a back pressure acting on the ink present in the recording head changes, which leads to lowering a recording quality of the recording head.
If the ink bag is assembled into the ink cartridge, not in the above-indicated upright posture but in a lying-down posture in which the ink bag takes a minimum height, an amount of change of the ink level of the ink bag and accordingly an amount of influence of that change to the back pressure can be reduced as compared with the case where the bag is assembled in its upright posture. However, in a full-color recording apparatus, a plurality of ink cartridges are arranged in an array in a widthwise direction thereof, which leads to increasing an overall size of the recording apparatus. On the other hand, in the case where it is required to reduce the overall size of the recording apparatus, a size of each ink bag (i.e., an amount of ink stored in each ink bag) and/or a position where each ink cartridge is attached are/is limited. In addition, in the case where the ink bag is formed of flexible sheets, those flexible sheets as a whole are not uniformly deformed as the amount of ink decreases. Therefore, if the deformation of the flexible sheets may directly be detected, or if an electrode may be inserted into the ink bag, it is difficult to detect, with accuracy, a remaining amount of the ink under the condition that the remaining amount is small.